Projection optical assembly with multiple refractive optical systems and related projector device

ABSTRACT

There is provided a projection optical system ( 1 ) that projects from a first image plane on a reducing side to a second image plane on an enlargement side, including a first refractive optical system ( 11 ) that includes eight lenses (L 1 ) to (L 8 ) and forms a first intermediate image ( 31 ) on the enlargement side using light that is incident from the reducing side, a second refractive optical system ( 12 ) that includes six lenses (L 9 ) to (L 14 ) and forms the first intermediate image ( 31 ) on the reducing side into a second intermediate image ( 32 ) on the enlargement side, and a first reflective optical system ( 20 ) that includes a first reflective surface ( 21   a ) with positive refractive power that is positioned closer to the enlargement side than the second intermediate image ( 32 ).

TECHNICAL FIELD

The present invention relates to a projection optical system of aprojector apparatus.

BACKGROUND ART

Japanese Laid-Open Patent Publication No. 2004-258620 (hereinafter“Document 1”) discloses the realization of a projection optical systemwhich, in addition to using an image forming optical system including areflective surface to increase the size on the screen of projectedimages while reducing the projection space outside a projectorapparatus, is capable of correcting chromatic aberration and also animage projecting apparatus that uses such projection optical system. Todo so, Document 1 discloses that a first and second optical system aredisposed in that order from a light valve on the projection side of thelight valve, the first optical system includes at least one refractiveoptical system and has positive refractive power, the second opticalsystem includes at least one reflective surface with refractive powerand has positive refractive power, an image formed by the light valve isformed into an intermediate image on the optical path of the first andsecond optical systems, and the intermediate image is enlarged furtherand projected onto a screen.

Japanese Laid-Open Patent Publication No. 2004-295107 (hereinafter“Document 2”) discloses a technology that realizes a variablemagnification optical system that is compact and achieves a desired zoomratio while suppressing the occurrence of various aberrations such aslateral chromatic aberration. The variable magnification optical systemin Document 2 is composed of an optical block R including threereflective curved surfaces and an optical block C disposed on thereducing side of the optical block R, the optical block C includes aplurality of movable lens units, and zooming is carried out by movingthe plurality of lens units. When tracing the optical axis from thereducing side to the enlargement side, the optical block C forms animage at a reducing side conjugate point that is closer to theenlargement side than the optical surface (reflective surface) closestto the reducing side in the optical block R.

In a variety of applications such as presentations and education inschools, there is demand for a projection lens system that is morecompact and capable of being made more wide angle.

DISCLOSURE OF THE INVENTION

A first aspect of the present invention is a projection optical systemthat projects from a first image plane on a reducing side to a secondimage plane on an enlargement side. The projection optical systemincludes: a first refractive optical system that includes a plurality oflenses and forms light incident from the reducing side into a firstintermediate image on the enlargement side; a second refractive opticalsystem that includes a plurality of lenses and forms the firstintermediate image on the reducing side into a second intermediate imageon the enlargement side; and a first reflective optical system thatincludes a first reflective surface with positive refractive power thatis positioned closer to the enlargement side than the secondintermediate image.

In this projection optical system, by having the first refractiveoptical system form the first intermediate image and the secondrefractive optical system form such first intermediate image into thesecond intermediate image on the enlargement side, it is possible toreduce the lens diameter on the enlargement side of the secondrefractive optical system. Accordingly, it is possible to make thesecond refractive optical system compact and additionally it is easy tomake an air gap (optical distance) from the second refractive opticalsystem to the first reflective surface shorter, which makes it possibleto make the first reflective surface smaller.

It is not easy to create a design where trapezoidal distortion (keystonedistortion) is corrected together with astigmatism and the like at thefirst reflective surface. With this projection optical system, it iseasy to have the first refractive optical system form the firstintermediate image where aberrations such as astigmatism are correctedand to have the second refractive optical system form the secondintermediate image where mainly keystone distortion and the like areadjusted. Accordingly, it is easy to project images that are sharp andin which keystone distortion has been corrected.

In this projection optical system, the first intermediate image, thesecond intermediate image, and the image formed by the first reflectivesurface may be respectively inverted. Accordingly, it is possible todesign the projection optical system so that when the first refractiveoptical system forms an image formed on the first image plane as thefirst intermediate image, a light ray that reaches a center of thesecond image plane from a center of the first image plane reaches thesecond image plane having crossed any of an optical axis of the firstrefractive optical system, an optical axis of the second refractiveoptical system, and an optical axis of the first reflective opticalsystem three times.

The light ray reaches the center of the second image plane from thecenter of the first image plane with crossing a common optical axisthree times if the optical axis of the first refractive optical system,the optical axis of the second refractive optical system, and theoptical axis of the first reflective optical system are common, or anyof such axes three times if the respective optical axes are shifted, andthe light ray crosses the optical axis twice between the first imageplane and the first reflective surface. The first image plane and thefirst reflective surface can be disposed in the same side (direction)with respect to the optical axis. That makes possible to dispose thefirst image plane and the first reflective surface in the same side withrespect to a first plane that includes the optical axis. This means thatit is possible to dispose an illumination optical system thatilluminates the first image plane in the same side as the firstreflective surface with respect to the first plane. Accordingly, it ispossible for the illumination optical system and the first reflectivesurface to share a space in the same side with respect to the firstplane. This means that it is possible to make a projector that includesthe projection optical system and the illumination optical systemslimmer or thin.

It is preferable for an effective diameter of a lens that is closest tothe enlargement side of the second refractive optical system that islocated on the enlargement side to be smaller than a maximum effectivediameter of the first refractive optical system that is located on thereducing side. It is more preferable for a maximum effective diameter ofthe second refractive optical system located on the enlargement side tobe smaller than a maximum effective diameter of the first refractiveoptical system located on the reducing side. A projection optical systembecomes compact and interference between light rays that reach the firstreflective surface and light rays reflected by the first reflectivesurface can be suppressed.

The first refractive optical system should preferably be an equalmagnification or a magnifying optical system. Although the firstintermediate image may be a reduced image, by magnifying or forming thefirst intermediate image with equal size, the magnification ratio of theoptical systems that are closer to the enlargement side than the firstrefractive optical system is relatively suppressed, which makes iteasier to correct aberration. It is possible to favorably correctvarious aberrations such as curvature of field, astigmatism, and comaticaberration using the first refractive optical system and to also correctdistortion such as keystone distortion using the second refractiveoptical system.

It is desirable for the projection optical system to include a stop thatis disposed between the first intermediate image and the secondintermediate image. It is also possible to make the size on theenlargement side of the second refractive optical system more compact.In addition, since it is possible to make the air gap (distance) betweenthe second refractive optical system and the first refractive opticalsystem shorter, it is possible to make the size of the first reflectivesurface much more compact.

In this aspect of the invention, the projection optical system couldinclude a first optical system that includes the first refractiveoptical system and the second refractive optical system, the firstintermediate image being formed inside the first optical system and thefirst optical system being divided into the first refractive opticalsystem and the second refractive optical system, and the exit pupil ofthe first optical system being closer to the first reflective surface.This means that it is possible to make the first reflective surfacesmaller. In this case, it is desirable for an optical distance EXPbetween an exit pupil and the first reflective surface in a case where afirst stop disposed between the first image plane and the firstintermediate image is as a stop of the first optical system and anoptical distance dw between the first image plane and the firstreflective surface satisfy the Condition (1) below.0.1<EXP/dw<0.6  (1)

In addition, by forming the first intermediate image inside the firstoptical system and dividing the first optical system into the firstrefractive optical system and the second refractive optical system, itis possible to dispose a second stop between the first intermediateimage and the second intermediate image to constrain the light flux.This means that it is possible to make the lens size on the enlargementside of the first optical system, and in particular the lens size on theenlargement side of the second refractive optical system smaller. Thesecond stop may be disposed inside the second refractive optical system.The second stop should preferably be an eccentric stop which makes itpossible to shut out scattered light such as flare and ghosts.

Also, since it is possible to make the lens size on the enlargement sideof the first optical system, that is, the enlargement side of the secondrefractive optical system smaller, it is possible to suppressinterference between the lenses and the light rays from the firstreflective surface, even when lenses that have rotational symmetry aboutthe optical axis are used. This means that it is not necessary todispose a lens with negative power on the enlargement side of the secondrefractive optical system to achieve sufficient distance between thesecond refractive optical system and the first reflective surface.Accordingly, the lens closest to the enlargement side of the secondrefractive optical system may be a positive lens or a positive meniscuslens, and may also be a cemented lens.

In addition, it is desirable for an optical distance do between the lensclosest to the enlargement side of the second refractive optical systemand the first reflective surface and an optical distance dw between thefirst image plane and the first reflective surface to satisfy theCondition (2) below0.1<dn/dw<0.3  (2).Since it is possible to reduce the space between the lenses included inthe second refractive optical system and the first reflective surface,the opening for emitting projected light between the lens and the firstreflective surface becomes smaller. Accordingly, it is possible toprovide a much more compact projector and to reduce the risk of damageto the lenses and the reflective surface due to dirt or dust that entersfrom the opening.

In this projection optical system, the first intermediate image and thesecond intermediate image are typically formed on opposite sides of theoptical axis. The first optical system may form one or a plurality ofintermediate images on the reducing side of the first intermediateimage. Also, the first reflective optical system may include one or aplurality of reflective surfaces before or after the first reflectivesurface. The projection optical system may also include a refractiveoptical system on the enlargement side of the first reflective opticalsystem.

Also, the first optical system may be a variable magnification opticalsystem (zoom optical system). It is desirable for the first opticalsystem to include, in order from the reducing side, a former group withpositive refractive power, a middle group with positive refractivepower, and a latter group with positive refractive power, and whenzooming from a wide angle end to a telephoto end, for the former groupto move from the reducing side to the enlargement side, for the middlegroup to move so as to compensate for movement of the former group, andfor the latter group to be fixed, and for the first intermediate imageto be formed inside the latter group. Light flux that is incident on thelatter group can be compensated to a certain state by the middle groupand movement of the first intermediate image during zooming can besuppressed. Accordingly, it is possible to zoom the image projected ontothe second image plane with hardly any movement in the positions of thefirst intermediate image and the second intermediate image.

Also, when zooming from the wide angle end to the telephoto end, it ispossible to change the magnification ratio of mainly the firstintermediate image by moving the former lens group and to correctvarious aberrations such as curvature of field, astigmatism, and comaticaberration via movement of the middle group. This means that it ispossible to provide a high resolution projection optical system thatincludes a variable magnification (variator) optical system thatsuppresses fluctuations in aberration and fluctuations in the positionof the first intermediate image.

Another aspect of the present invention is a projector including: theprojection optical system described above; and a light modulator thatforms an image on the first image plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an arrangement of projector apparatuses that use aprojection optical system according to the present invention, with FIG.1(a) showing an example of a projector apparatus that uses anon-telecentric projection optical system, FIG. 1(b) showing an exampleof a projector apparatus that uses a telecentric projection opticalsystem, and FIG. 1(c) showing an example of a projector apparatus thatuses a different telecentric projection optical system.

FIG. 2 shows an arrangement of a projection optical system according toa first embodiment.

FIG. 3 shows lens data of the projection optical system according to thefirst embodiment.

FIG. 4 shows various numeric values of the projection optical systemaccording to the first embodiment, with (a) showing fundamental data,(b) showing gap data, and (c) showing aspherical surface data.

FIG. 5 shows a state of light flux on planes in the peripheries ofintermediate images in the projection optical system according to thefirst embodiment, with (a) showing the periphery of the firstintermediate image and (b) showing the periphery of the secondintermediate image.

FIG. 6 shows an arrangement of a projection optical system according toa second embodiment.

FIG. 7 shows lens data of the projection optical system according to thesecond embodiment.

FIG. 8 shows various numeric values of the projection optical systemaccording to the second embodiment, with (a) showing fundamental data,(b) showing gap data, (c) showing aspherical surface data, and (d)showing zoom data.

FIG. 9 shows a state of light flux on planes in the peripheries ofintermediate images in the projection optical system according to thesecond embodiment, with (a) showing the periphery of the firstintermediate image and (b) showing the periphery of the secondintermediate image.

FIG. 10 shows an arrangement of a projection optical system according toa third embodiment.

FIG. 11 shows lens data of the projection optical system according tothe third embodiment.

FIG. 12 shows various numeric values of the projection optical systemaccording to the third embodiment, with (a) showing fundamental data,(b) showing gap data, (c) showing aspherical surface data, and (d)showing zoom data.

FIG. 13 shows a state of light flux on planes in the peripheries ofintermediate images in the projection optical system according to thethird embodiment, with (a) showing the periphery of the firstintermediate image and (b) showing the periphery of the secondintermediate image.

FIG. 14 shows an arrangement of a projector apparatus that uses aconventional projection optical system.

DETAIL DESCRIPTION

FIG. 1 shows a general configuration of projector apparatuses that usesa typical projection optical system according to an embodiment of thepresent invention, FIG. 1(a) is a diagram showing a configuration of aprojector apparatus that uses a non-telecentric projection opticalsystem, FIG. 1(b) is a diagram showing a configuration of a projectorapparatus that uses a telecentric projection optical system, and FIG.1(c) is a diagram showing a configuration of a projector apparatus thatuses a different telecentric projection optical system. FIG. 14 shows ageneral configuration of a projector apparatus that uses a conventionalprojection optical system.

As shown in FIGS. 1(a) to 1(c), the projector (projector apparatus) 100includes a light modulator (light valve) 5, an illumination (lighting)optical system 90 that illuminates the light valve 5 with illuminationlight to be modulated, and a projection optical system 1 that enlargesand projects an image formed by the light valve 5 on the light valve 5that may be a first image plane on the reducing side, with projectinglight 91 onto a screen 9 that is a second image plane on the enlargementside. The light valve 5 may be a device capable of forming an image suchas an LCD, a digital mirror device (DMD) or an organic EL display, andmay be a single panel-type device or a device that uses a method whereimages of different colors are individually formed. Note that the lightvalve 5 described above may be a reflective LCD or a transmissive LCD.If the light valve 5 is a transmissive-type, the illumination opticalsystem 90 is disposed on the opposite side of the light valve 5 in thedirection of an optical axis 101 of the projection optical system 1. Thescreen 9 may be a wall surface, a white board, or the like. Theprojector 100 may be a front projector, or a rear projector thatincludes a screen. Note that the light valve 5 indicates that positionof the first image plane of the light valve.

A typical light valve 5 is a single panel-type video projector that usesa DMD (digital mirror device). The illumination optical system 90includes a white light source, such as a halogen lamp, and a rotatingcolor splitting filter (color wheel) in the form of a disc, with the DMD(panel, light valve) 5 forming images in the three colors red, green,and blue according to time division. The DMD 5 side of the projectionoptical system 1 may be non-telecentric as shown in FIG. 1(a), but canalso be made telecentric using a TIR (Total Internal Reflection) prismPr as shown in FIG. 1(b). Note that when a liquid crystal display isused instead of a DMD, it is also possible to use a color combiningprism in place of the TIR prism as shown in FIG. 1(c). When a reflectiveliquid crystal display is used, it is also possible to use both anillumination prism or wire grid and a color combining prism

As shown in FIGS. 1(a) to 1(c), the projection optical systems 1according to the present invention includes a projection optical systemthat projects from the DMD 5 that is a first image plane on the reducingside onto the screen 9 that is the second image plane on the enlargementside. The projection optical system 1 includes a first optical system 10that includes a plurality of lenses and forms a first intermediate image31, which has been formed inside the first optical system 10 by lightthat is incident from the reducing side, into a second intermediateimage 32 that is closer to the enlargement side than the first opticalsystem 10, and a second optical system (first reflective optical system)20 that includes a first reflective surface 21 a with positiverefractive power positioned closer to the enlargement side than thesecond intermediate image 32. The first optical system 10 can bearranged so as to include a first refractive optical system 11 thatforms the first intermediate image 31 and a second refractive opticalsystem 12 that forms the first intermediate image 31 into the secondintermediate image 32.

In this projection optical system 1, the first intermediate image 31,the second intermediate image 32, and the image focused by the firstreflective surface 21 a are respectively inverted. Accordingly, it ispossible to design the projection optical system 1 so that a light raythat reaches the center of the screen 9 from the center of the DMD 5reaches the screen 9 having crossed the optical axis 101 that is commonto the first optical system 10 and the second optical system 20 threetimes.

If light rays that reach the center of the screen 9 from the center ofthe DMD 5 reach the screen 9 having crossed the optical axis 101 that iscommon to the refractive optical system 11, the second refractiveoptical system 12, and the second optical system 20 three times, thelight rays will cross the optical axis 101 twice between the DMD 5 andthe first reflective surface 21 a. Accordingly, the DMD 5 and the firstreflective surface 21 a can be disposed in the same direction withrespect to the optical axis 101. That is, the DMD 5 and the firstreflective surface 21 a can be disposed in a first direction 111 a (thedownward side) with respect to a first plane 111 that includes theoptical axis 101.

In the same way, the illumination optical system 90 that illuminates theDMD 5 can be disposed in the first direction 111 a (i.e., the same sideas the first reflective surface 21 a) with respect to the first plane111. Accordingly, unlike a conventional projector apparatus 200 such asthat shown in FIG. 14, it is possible for the illumination opticalsystem 90 and the first reflective surface 21 a to commonly utilize aspace 111 s in the same first side 111 a with respect to the first plane111. This means that the height (thickness) of the separating apparatus100 including the projection optical system 1 and the illuminationoptical system 90 can be reduced to half or less than the conventionalapparatus 200.

As shown in FIG. 1(a) to FIG. 1(c), in the projector 100 including theprojection optical system 1 according to the present invention, theillumination optical system 90 and the first reflective surface 21 a canbe disposed in the space 111 s in the same side of the first direction111 a with respect to the first plane 111. This means that it ispossible to fit the illumination optical system 90 into the height(thickness) h in which the first reflective surface 21 a fits. Also,even if the thickness of the illumination optical system 90 is large, itwill be possible to fit the first reflective surface 21 a into thethickness h of the illumination optical system 90.

First Embodiment

FIG. 2 shows the projection optical system 1 according to a firstembodiment. This projection optical system 1 is a fixed focus-type(single focus-type) projection optical system that is telecentric on theincident side. The projection optical system 1 includes, in order fromthe side of the DMD 5, which is the first image plane on the reducingside, the first optical system 10 that includes a plurality of lensesand the second optical system (first reflective optical system) 20 thatincludes the first reflective surface 21 a that has positive refractivepower and projects light emitted from the first optical system 10 ontothe screen 9 that is the second image plane on the enlargement side.More specifically, the first optical system 10 includes fourteen lensesL1 to L14. The second optical system 20 includes a single mirror(concave mirror) 21 including the first reflective surface 21 a in theform of an aspherical surface. The projection optical system 1 in thepresent embodiment is a single focus (fixed focus) type optical systemwhere zooming is not carried out. In this projection optical system 1,light projected onto the screen 9 that is the second image plane by theplurality of lenses L1 to L14 of the first optical system 10 and themirror 21 of the second optical system 20 enlarges and projects an imageformed on the DMD 5 (that is, the first image plane) onto the screen 9.

The first optical system 10 of this projection optical system 1 formsthe first intermediate image 31, which is formed inside the firstoptical system 10 by light incident from the DMD 5, into the secondintermediate image 32 that is closer to the enlargement side than thefirst optical system 10. The first reflective surface 21 a of the secondoptical system 20 is also disposed closer to the enlargement side thanthe second intermediate image 32. The first optical system 10 shown inFIG. 2 is a refractive optical system that does not include a mirrorsurface. This first optical system 10 includes the first refractiveoptical system 11 that forms an image formed by the DMD 5 into the firstintermediate image 31 and the second refractive optical system 12 thatforms the first intermediate image 31 into the second intermediate image32. Note that the first optical system 10 may also include a mirrorsurface for bending the optical axis 101 at an appropriate position.

The first refractive optical system 11 is a lens system which as a wholehas positive refractive power. The first refractive optical system 11 iscomposed, in order from the DMD 5 side, of a biconvex positive lens L1,a positive meniscus lens L2 that is convex on the DMD 5 side, a firstcemented lens (balsam lens, doublet) LB1 where two lenses are stucktogether, a negative meniscus lens L5 that is convex on the mirror 21side (enlargement side), a positive meniscus lens L6 that is convex onthe mirror 21 side, a positive meniscus lens L7 that is convex on theDMD 5 side (reducing side), and a positive meniscus lens L8 that isbiconvex and made of resin. The first cemented lens LB1 is composed of abiconvex positive lens L3 and a biconcave negative lens L4 that aredisposed in that order from the DMD 5 side. Both surfaces of thenegative meniscus lens L5, that is, the surface S8 on the DMD 5 side andthe surface S9 on the mirror 21 side, are aspherical surfaces. Also,both surfaces of the positive meniscus lens L8, that is, the surface S14on the DMD 5 side and the surface S15 on the mirror 21 side, areaspherical surfaces.

The positive meniscus lens L7 that is second closest to the enlargementside out of the first refractive optical system 11 is the lens with thelargest effective diameter (aperture) out of the lenses of the firstrefractive optical system 11 and is the lens that gives the maximumeffective diameter of the first refractive optical system 11. At thesame time, the positive refractive lens L7 is the lens with the largesteffective diameter (aperture) out of the lenses of the first opticalsystem 10, so that the maximum effective diameter of the first opticalsystem 10 is given by a lens on the enlargement side of the firstrefractive optical system 11.

A first aperture stop St1 that forms the first intermediate image 31 isdisposed in a space on the mirror 21 side of the negative meniscus lensL5, that is, between the negative meniscus lens L5 and the positivemeniscus lens L6. A single glass prism (TIR prism) Pr is disposed on theDMD 5 side of the first refractive optical system 11 so that the lightincident on the projection optical system 1 becomes telecentric or astate that is near-telecentric. The first refractive optical system 11forms the image formed by the DMD 5 into the first intermediate image 31closer to the enlargement side than the first refractive optical system11, that is, in the space Sp1 between the first refractive opticalsystem 11 and the second refractive optical system 12. The firstintermediate image 31 in the present embodiment is formed so as to beseparated by an air gap (distance) of 3.74 mm on the enlargement sidefrom the positive meniscus lens L8 that is closest to the enlargementside out of the lenses in the first refractive optical system 11.

The second refractive optical system 12 is a lens system which as awhole has a high positive refractive power and a shorter focal length(focal distance) than the first refractive optical system 11. The secondrefractive optical system 12 is composed of a positive meniscus lens L9that is made of resin and is convex on the DMD 5 side, a biconvexpositive lens L10, a second cemented lens LB2 where two lenses are stucktogether, and a third cemented lens LB3 where two lenses are stucktogether, disposed in that order from the DMD 5 side. The secondcemented lens LB2 is composed of a biconvex positive lens L11 and abiconcave negative lens L12 that are disposed in that order from the DMD5 side. The third cemented lens LB3 is composed of a negative meniscuslens L13 that is convex on the DMD 5 side and a biconvex positive lensL14 that are disposed in that order from the DMD 5 side. Both surfacesof the positive meniscus lens L9, that is, the surface S16 on the DMD 5side and the surface S17 on the mirror 21 side, are aspherical surfaces.

The positive meniscus lens L9 that is closest to the reducing side ofthe second refractive optical system 12 is the lens with the largesteffective diameter (aperture) out of the second refractive opticalsystem 12 and is the lens that provides the maximum effective diameterof the second refractive optical system 12. A second eccentric stop St2that forms the second intermediate image 32 is disposed on the mirror 21side of the second refractive optical system 12. The second eccentricstop St2 according to the present embodiment has the center of anaperture (a circular opening) displaced from the optical axis 101 andthe degree of eccentricity of such opening from the center is 1.5 mm inthe downward direction (first direction) 111 a with respect to the firstplane 111 that includes the optical axis 101. The second refractiveoptical system 12 forms the second intermediate image 32 closer to theenlargement side than the second refractive optical system 12, that is,in a space Sp2 between the second refractive optical system 12 and thefirst reflective surface 21 a. The second intermediate image 32 in thepresent embodiment is formed on the mirror 21 side of the secondeccentric stop St2 so as to be separated by an air gap of 41.60 mm fromthe second eccentric stop St2.

In this projection optical system 1, the first refractive optical system11 forms the first intermediate image 31 in the space Sp1 inside thefirst optical system 10 and the second refractive optical system 12forms the second intermediate image 32 in the space Sp2 on theenlargement side of the first optical system 10 and on the reducing sideof the first reflective surface 21 a of the second optical system 20. Inaddition, the aspherical reflective surface 21 a of the second opticalsystem 20 projects the second intermediate image 32 onto the screen 9 toenlarge and project the image of the DMD 5 onto the screen 9.

In this projection optical system 1, the second refractive opticalsystem 12 disposed closest to the enlargement side of the first opticalsystem 10 forms the first intermediate image 31 formed inside the firstoptical system 10 as the second intermediate image 32 on the enlargementside of the second refractive optical system 12. The first intermediateimage 31 and the second intermediate image 32 are inverted with respectto the optical axis 101. Accordingly, light flux that passes the secondrefractive optical system 12 crosses the optical axis 101 of the secondrefractive optical system 12 and the area of the light flux that passesthe second refractive optical system 12 tends to be centered on theperiphery of the optical axis 101. This means that the maximum effectiveaperture of the second refractive optical system 12 positioned on theenlargement side of the first optical system 10 can be made smaller thanthe first refractive optical system 11. In particular, the lens diameteron the enlargement side of the second refractive optical system 12 canbe made smaller relative to the lens diameter on the reducing side.

That is, the first refractive optical system 11 forms the firstintermediate image 31 in one region (first region) 101 a that is anupper half relative to the optical axis 101 and the second refractiveoptical system 12 forms the second intermediate image 32, where up-downand left-right are inverted relative to the first intermediate image 31,in another region (second region) 101 b that is opposite the region 101a, that is, the lower half relative to the optical axis 101. This meansthat the light flux that reaches the second intermediate image 32 fromthe first intermediate image 31 is concentrated in the periphery of theoptical axis 101 and the second refractive optical system 12 and thefirst reflective surface 21 a can be made smaller.

In addition, in the projection optical system 1, a light ray or lightrays 110 that reach the center of the second image plane of the screen 9from the center of the first image plane formed on the DMD 5 reach thescreen 9 having crossed the optical axis 101 that is common to the firstoptical system 10 and the second optical system 20 three times. Morespecifically, the light rays 110 emitted from the DMD 5 are incident onthe projection optical system 1 from below (in the FIG. 2) the opticalaxis 101, cross the optical axis 101 inside the first refractive opticalsystem 11, and are formed into the first intermediate image 31 above theoptical axis 101. Then, the light rays 110 cross the optical axis 101inside the second refractive optical system 12 and are formed into thesecond intermediate image 32 below the optical axis 101. The light rays110 are also reflected by the first reflective surface 21 a below theoptical axis 101 and are projected onto the screen 9 after crossing theoptical axis 101 once again. Note that “above” and “below” the opticalaxis 101 are relative positional relationships and that above and belowthe optical axis 101 may be interchanged, or may be to the left andright of the optical axis 101.

The first optical system 10 forms the first intermediate image 31internally and forms such first intermediate image 31 into the secondintermediate image 32 on the enlargement side. This means that it ispossible to provide the eccentric stop St2 for forming the secondintermediate image 32 formed on the enlargement side at a positioncloser the enlargement side within the first optical system 10 or closerto the enlargement side than the first optical system 10. Accordingly,it is possible to reduce the air gap between the second intermediateimage 32 formed on the reducing side of the first reflective surface 21a and the eccentric stop St2. This means that it is possible to providesufficient widening for the light flux that reaches the first reflectivesurface 21 a from the second intermediate image 32 relative to the sizeof the second intermediate image 32. Accordingly, it is possible toreduce the lens diameter on the enlargement side of the secondrefractive optical system 12 and to reduce the diameter of surface ofthe first reflective surface 21 a that has rotational symmetry about theoptical axis 101.

This projection optical system can be designed so that the opticaldistance EXP between the exit pupil of the first optical system 10 andthe first reflective surface 21 a and the optical distance dw betweenthe DMD and the first reflective surface 21 a satisfy Condition (1)below.0.1<EXP/dw<0.6  (1)

By setting the distance EXP between the exit pupil of the first opticalsystem 10 and the first reflective surface 21 a in a range thatsatisfies Condition (1), it is possible to bring the exit pupil of thefirst optical system closer to the first reflective surface 21 a. Thismeans that it is possible to make the first reflective surface 21 asmaller. When the upper limit in Condition (1) is exceeded, the air gapbetween the lens on the enlargement side of the first optical system 10and the first reflective surface 21 a becomes longer, which makes itdifficult to make the first reflective surface 21 a smaller. When thelower limit in Condition (1) is exceeded, it is difficult to suppressinterference between light that has been reflected by the firstreflective surface 21 a toward the screen 9 and the second refractiveoptical system 12. It is desirable for the upper limit in Condition (1)to be 0.4. It is desirable for the lower limit in Condition (1) to be0.2, with 0.24 being even more desirable.

In addition, since it is possible to reduce the lens diameter on theenlargement side of the second refractive optical system 12, it ispossible to suppress interference between light that has been reflectedby the first reflective surface 21 a toward the screen 9 and the secondrefractive optical system 12 even if the second refractive opticalsystem 12 is formed of lenses that have surfaces with rotationalsymmetry centered on the optical axis 101. Accordingly, it is possibleto provide the projection optical system 1 that is capable of reducingthe air gap between the second refractive optical system 12 and thefirst reflective surface 21 a, has a compact overall construction, andis capable being made more wide angle.

The projection optical system 1 can be designed so that the distance dnbetween the enlargement side of the first optical system 10, that is, alens (in the present embodiment, the positive lens L14) disposed closestto the mirror 21 and the first reflective surface 21 a and the distancedw between the DMD 5 and the first reflective surface 21 a satisfyCondition (2) below.0.1<dn/dw<0.3  (2)

By setting the distance dn between the first optical system 10 and thefirst reflective surface 21 a in the range of Condition (2), it ispossible to reduce the space Sp2 between the first optical system 10 andthe first reflective surface. This makes it easier to suppressmechanical damage to the lens on the wide angle side of the firstoptical system 10 (the positive lens L14) and the first reflectivesurface 21 a. If the upper limit in Condition (2) is exceeded, the spaceSp2 between the lens L14 on the wide angle side and the first reflectivesurface 21 a becomes relatively large, which increases the risk ofreceiving mechanical damage. If the lower limit in Condition (2) isexceeded, the space Sp2 becomes small, the first reflective surface 21 abecomes too close to the second intermediate image 32, and it is notpossible to make system sufficiently more wide angle. It is desirablefor the upper limit of Condition (2) to be 0.26. It is also desirablefor the lower limit of Condition (2) to be 0.15.

It is possible to set the ratio of the effective diameter MD of thefirst reflective surface 21 a to the image circle IC on the reducingside of the projection optical system 1 as shown in Condition (3) below.1.0≦MD/IC≦6.0  (3)The upper limit of Condition (3) may be 5.0, with 4.5 being even morepreferable. Also, the lower limit of Condition (3) may be 2.0, with 2.5being even more preferable. It is possible to provide the projectionoptical system 1 which enables the first reflective surface 21 a to bemade smaller relative to the size of the image circle IC and which iseven more compact.

It is also possible to set the ratio of the effective diameter LLD ofthe lens closest to the enlargement side out of the first optical system10 (in the present embodiment, the lens L14) to the image circle IC onthe reducing side of the projection optical system 1 as shown inCondition (4) below.0.1≦LLD/IC≦2.0  (4)The upper limit of Condition (4) may be 1.5, with 1.0 being even morepreferable. Also, the lower limit of Condition (4) may be 0.2, with 0.3being even more preferable. It is possible to provide the projectionoptical system 1 where, by making the lens diameter (effective diameter)closest to the enlargement side of the first optical system 10 smallerrelative to the size of the image circle IC, it is possible to preventinterference between the light flux (projected light) reflected by thefirst reflective surface 21 a and the lenses, and which is even compact.

It is possible to set the ratio of the diameter STD2 of the eccentricstop St2 to the effective diameter MD of the first reflective surface 21a of the projection optical system 1 as shown in Condition (5) below.1.0≦MD/STD2≦30  (5)The upper limit of Condition (5) may be 25, with 20 being preferable and18 being even more preferable. Also, the lower limit of Condition (5)may be 2.0, with 3.0 being desirable and 4.0 being even more desirable.By providing the eccentric stop St2 on the enlargement side of theprojection optical system 1 to set the effective diameter MD of thefirst reflective surface 21 a in the range given above, it is possibleto make the effective diameter MD smaller.

The maximum effective diameter of the first refractive optical system 11is the effective diameter of the positive meniscus lens L7 (in thepresent embodiment, 49.0 mm), and the effective diameter of the positivelens L14 that is closest to the enlargement side of the secondrefractive optical system 12 is 17.0 mm in the present embodiment. Themaximum effective diameter of the second refractive optical system 12 isthe effective diameter of the positive meniscus lens L9 (in the presentembodiment, 36.0 mm). Accordingly, the effective diameter of the lensL14 that is closest to the enlargement side of the second refractiveoptical system 12 is smaller than the maximum effective diameter of thefirst refractive optical system 11 and also the maximum effectivediameter of the second refractive optical system 12 is smaller than themaximum effective diameter of the first refractive optical system 11.Accordingly, in the first optical system 10 as a whole, the lensdiameter is smaller on the enlargement side than on the reducing sideand in the second refractive optical system 12 also, the lens diameteris smaller on the enlargement side than on the reducing side.

The first optical system 10 includes the first refractive optical system11 and the second refractive optical system 12 and is constructed sothat light rays are relayed via the first intermediate image 31.Accordingly, it is possible to construct the respective refractiveoptical systems 11 and 12 so that a sharp image is projected onto thescreen 9. That is, when looking from the enlargement side (wide angleside), the first reflective surface 21 a produces keystone distortion(trapezoidal distortion) and it is difficult to design the firstreflective surface 21 a so as to correct keystone distortion along withastigmatism and the like. In this system, the second intermediate image32 is assumed to have keystone distortion, and the first intermediateimage 31 in which aberrations such as curvature of field, astigmatism,and comatic aberration have been corrected is formed by the firstrefractive optical system 11, and an image where mainly keystonedistortion or the like of the first intermediate image 31 has beenadjusted by the second refractive optical system 12, that is, the secondintermediate image 32 where distortion has been produced in the oppositedirection (cancelling direction) to the distortion that will be producedby the first reflective surface 21 a is formed. Accordingly, theprojection optical system 1 including the first optical system 10 andthe second optical system 20 is capable of projecting sharp images onwhich keystone correction has been carried out onto the screen 9.

In this way, the projection optical system 1 includes, on both sides ofthe first intermediate image 31, the first refractive optical system 11on the reducing side (the DMD 5 side) of the first intermediate image 31and the second refractive optical system 12 on the enlargement side (themirror 21 side) of the first intermediate image 31. This means that byforming the first intermediate image 31 using the first refractiveoptical system 11, it is possible to correct aberrations such ascurvature of field, astigmatism, and comatic aberration and by alsoforming the second intermediate image 32 using the second refractiveoptical system 12, it is possible to adjust for keystone distortion andthereby correct distortion. Accordingly, by using a configuration withthe first intermediate image 31 in between the two refractive opticalsystems 11 and 12, it becomes possible to design the first refractiveoptical system 11 and the second refractive optical system 12 asrespectively dedicated optical systems. This means that there isincreased freedom when designing the respective refractive opticalsystems 11 and 12.

The positive meniscus lens L8 that is biconvex, made of resin, anddisposed on the reducing side of the first intermediate image 31 andclosest to the enlargement side of the first refractive optical system11 has low power, and both surfaces S14 and S14 are aspherical surfaces.This means that it is possible to favorably correct various aberrationsand to suppress a drop in the MTF of the first intermediate image 31.Also, the surface S16 on the reducing side of the positive meniscus lensL9 that is convex on the reducing side, made of resin, and disposedclosest to the reducing side of the second refractive optical system 12,that is, on the enlargement side of the first intermediate image 31 hasthe smallest radius of curvature (that is, a large curvature) out of thefirst optical system 10, and the surface S17 on the enlargement side hasthe next smallest radius of curvature (that is, a large curvature) afterthe surface S16. This means that it is easy to adjust the keystonedistortion of the first intermediate image 31 and to form the secondintermediate image 32 that includes been keystone distortion. Inaddition, since both surfaces S16 and S17 of the positive meniscus lensL9 are aspherical surfaces, correction of various aberrations aside fromkeystone distortion can be carried out at the same time. Accordingly, byusing a simple construction of disposing the positive meniscus lens L9made of resin closest to the DMD 5 side of the second refractive opticalsystem 12, it is possible to obtain, at low cost, the secondintermediate image 32 where various aberrations aside from keystonedistortion are suppressed.

FIG. 3 shows lens data of the lenses of the first optical system 10 ofthe projection optical system 1. FIG. 4 shows various numeric values ofthe projection optical system 1. In the lens data, “Ri” represents theradius of curvature (mm) of each lens (i.e., each lens surface) disposedin order from the DMD (light valve) 5 side (the reducing side), “di”represents the distance (mm) between the respective lens surfacesdisposed in order from the DMD 5 side, “Di” represents the effectivediameter (mm) of each lens surface disposed in order from the DMD 5side, “nd” represents the refractive index (d line) of each lensdisposed in order from the DMD 5 side, and “vd” represents the Abbenumber (d line) of each lens disposed in order from the DMD 5 side. InFIG. 3, “Flat” indicates a flat surface. In FIG. 4(c), “En” represents“10 to the power n” and as one example, “E-06” represents “10 to thepower −6”. The same also applies to the following embodiments. In thepresent specification, the position of the first intermediate image 31is shown by the focus point position of the light flux of the firstintermediate image 31 on the optical axis 101. The position of thesecond intermediate image 32 is shown as the averaged position of theoptical distance d1 from the second eccentric stop St2 to the secondintermediate image 32 on the optical axis 101 and the optical distanced2 between the second eccentric stop St2 and the closest periphery(closest edge) of the second intermediate image 32. In the presentembodiment, since d1 is 58.20 mm and d2 is 25.00 mm, the position of thesecond intermediate image 32 is shown at the position 41.60 mm from thesecond eccentric stop St2. This is also the same as the followingembodiments.

FIG. 5 shows the light flux that crosses a flat plane in the peripheriesof the first intermediate image 31 and the second intermediate image 32of the projection optical system 1 by way of spot diagrams. As shown inFIG. 5(a), at the periphery of the first intermediate image 31, theimage formed by the DMD 5 has been inverted in the up-down andleft-right directions as an image that has been enlarged by the firstrefractive optical system 11. As shown in FIG. 5(b), at the periphery ofthe second intermediate image 32, the first intermediate image 31 isinverted in the up-down and left-right directions as an image that haskeystone distortion due to the second refractive optical system 12.

Since the values in the equation given as Condition (1) described aboveof this projection optical system 1 is such that the optical distanceEXP between the exit pupil of the first optical system 10 and the firstreflective surface 21 a is 81.70 mm and the optical distance dw betweenthe DMD 5 and the mirror 21 is 323.00 mm as shown in FIG. 4(b), theresult shown below is produced. Since the values in the equation givenas Condition (2) described above of this projection optical system 1 issuch that the distance dn between the positive lens L14 and the mirror21 is 75.20 mm and the distance dw between the DMD 5 and the mirror 21is 323.00 mm as shown in FIG. 4(b), the result shown below is produced.Also, the other Conditions (3) to (5) are as indicated below. Note thatthe position of the exit pupil of the first optical system 10 shows theposition of the exit pupil when the first stop St1 on the reducing sideis used as the stop of the first optical system 10.EXP/dw=0.25  Condition (1)dn/dw=0.23  Condition (2)MD/ID=2.8  Condition (3)LLD/ID=0.6  Condition (4)STD2/MD=4.7  Condition (5)Accordingly, the projection optical system 1 according to the presentembodiment satisfies Conditions (1) to (5).

As shown in the above, by being constructed of the fourteen lenses L1 toL14 and a single mirror 21, the projection optical system 1 according tothe present embodiment is one example of a high-performance projectionoptical system 1 that has a fixed focus point but is comparatively wideangle with a maximum angle of view (full angle) of 66.67 degrees and afocal length of 6.20, and is capable, with an F number of 1.90, ofprojecting bright, sharp images.

Note that both surfaces S8 and S9 of the negative meniscus lens L5 ofthe first refractive optical system 11, both surfaces S14 and S15 of thepositive meniscus lens L8 of the first refractive optical system 11,both surfaces S16 and S17 of the positive meniscus lens L9 of the secondrefractive optical system 12, and the first reflective surface 21 a areaspherical surfaces that exhibit rotational symmetry. The asphericalsurfaces are expressed by the following expression using thecoefficients K, A, B, C, and D shown in FIG. 4(c) with X as thecoordinate in the optical axis direction, Y as the coordinate in adirection perpendicular to the optical axis, the direction in whichlight propagates as positive, and R as the paraxial radius of curvature.This is also the case for the embodiments described later.X=(1/R)Y ²/[1+{1−(1+K)(1/R)² Y ²}^(1/2) ]+AY ⁴ +BY ⁶ +CY ⁸ +DY ¹⁰ +EY ¹²+FY ¹⁴

Second Embodiment

FIG. 6 shows a projection optical system 2 according to a secondembodiment. The projection optical system 2 is a projection opticalsystem that is non-telecentric on the incident side and is capable ofzooming. The projection optical system 2 includes, in order from the DMD5 side (the reducing side), the first optical system 10, which includesa plurality of lenses, and the second optical system (first reflectiveoptical system) 20 that includes the first reflective surface 21 a thatreflects light emitted from the first optical system 10 to project animage onto the screen 9. The first optical system 10 includes thirteenlenses L11 to L13, L21 to L22, and L31 to L38, and the second opticalsystem 20 includes a mirror (curved mirror) 21 on which an asphericalfirst reflective surface 21 a is formed.

This projection optical system 2 is a zoom-type (variable magnificationtype) optical system that carries out zooming. The first optical system10 includes, in order from the DMD 5 side, a first lens group (formergroup) G1 that has positive refractive power, a second lens group(middle group) G2 that has positive refractive power, and a third lensgroup (latter group) G3 that has positive refractive power. Also, thefirst optical system 10 according to the present embodiment is anoptical system that forms the first intermediate image 31 formed insidethe first optical system 10 into the second intermediate image 32 thatis closer to the enlargement side than the first optical system 10. Thefirst optical system 10 includes the first refractive optical system 11that has positive power and forms the image formed by the DMD 5 into thefirst intermediate image 31 and the second refractive optical system 12that has positive power and forms the first intermediate image 31 intothe second intermediate image 32. The first intermediate image 31 isformed inside the third lens group G3, with the first lens group G1, thesecond lens group G2, and the first lens (on the reducing side) L31 ofthe third lens group G3 configure the first refractive optical system11, with the other lenses of the third lens group G3 configuring thesecond refractive optical system 12.

The first lens group (former group) G1 closest to the DMD 5 (closest tothe reducing side) is a lens group that as a whole has positiverefractive power. The first lens group G1 is composed of a biconvexpositive lens L11, a biconcave negative lens L12, and a biconvexpositive lens L13 disposed in that order from the DMD 5 side. Bothsurfaces of the positive lens L11, that is, the surface S1 on the DMD 5side and the surface S2 on the mirror 21 side, are aspherical surfaces.A first aperture stop St1 is disposed in a space on the mirror 21 sideof the positive lens L11, that is, in a space between the positive lensL11 and the negative lens L12. A single cover glass CG made of glass isdisposed on the DMD 5 side of the first lens group G1.

The second lens group (middle group) G2 is a lens group that as a wholehas positive refractive power. The second lens group G2 is composed of afirst cemented lens (balsam lens, doublet) LB1 where two lenses arestuck together. The first cemented lens LB1 is constructed of a negativemeniscus lens L21 that is convex on the DMD 5 side and a positivemeniscus lens L22 that is convex on the DMD 5 side which are disposed inthat order from the DMD 5 side.

The third lens group (latter group) G3 closest to the mirror 21 side(the enlargement side) is a lens group that as a whole has positiverefractive power. The third lens group G3 is constructed of a positivemeniscus lens L31 that is made of resin and convex on the DMD 5 side, apositive meniscus lens L32 that is made of resin and is convex on theDMD 5 side, a positive meniscus lens L33 that is convex on the mirror 21side, a second cemented lens (balsam lens, triplet) LB2 where threelenses are stuck together, and a third cemented lens (balsam lens,doublet) LB3 where two lenses are stuck together, disposed in that orderfrom the DMD 5 side.

The second cemented lens LB2 is constructed of a biconvex positive lensL34, a biconcave negative lens L35, and a positive meniscus lens L36that is convex on the DMD 5 side disposed in that order from the DMD 5side. The third cemented lens LB3 is constructed of a biconvex positivelens L37 and a biconcave negative lens L38 disposed in that order fromthe DMD 5 side. Both surfaces of the positive meniscus lens L31, thatis, the surface S10 on the DMD 5 side and the surface S11 on the mirror21 side, are aspherical surfaces. In addition, both surfaces of thepositive meniscus lens L32, that is the surface S12 on the DMD 5 sideand the surface S13 on the mirror 21 side are aspherical surfaces. Asecond eccentric stop St2 is disposed on the mirror 21 side of thepositive meniscus lens L36, that is in the space between the positivemeniscus lens L36 and the positive lens L37.

In this projection optical system 2, when zooming from the wide angleend to the telephoto end, the first lens group (former group) G1 movesfrom the reducing side (the DMD 5 side) to the enlargement side (themirror 21 side), the second lens group (middle group) G2 also moves fromthe reducing side to the enlargement side, and the third lens group(latter group) G3 does not move. The first lens group G1 zooms by movingas a variator, the second lens group G2 moves as a compensator so as tocompensate for the movement of the first lens group G1 so that the lightflux incident on the third lens group G3 that is the relay lenssatisfies certain conditions. This projection optical system 2 is afloating focus or inner focus type optical system that adjusts the focuspoint inside the third lens group G3 that does not move during zooming.Focusing in the present embodiment is carried out by moving at least onelens included in the third lens group G3.

The first refractive optical system 11, which is composed of the firstlens group G1, the second lens group G2, and the lens L31, includes thepositive lens L11, the first stop SU, the negative lens L12, thepositive lens L13, the first cemented lens LB1, and the positivemeniscus lens L31, disposed in that order from the DMD 5 side, and thefirst intermediate image 31 is formed on the enlargement side of thefirst refractive optical system 11, that is, in the space Sp1 betweenthe first refractive optical system 11 and the second refractive opticalsystem 12. The first intermediate image 31 in the present embodiment isformed so as to be separated by an air gap (air distance) of 15.00 mmfrom the first meniscus lens L31 on the enlargement side.

The second refractive optical system 12 constructed of the third lensgroup G3 aside from the lens L31 includes the positive meniscus lensL32, the positive meniscus lens L33, the second cemented lens LB2, thesecond stop St2, and the third cemented lens LB3, disposed in that orderfrom the DMD 5 side. The second intermediate image 32 is formed on theenlargement side of the second refractive optical system 12, that is, inthe space Sp2 between the second refractive optical system 12 and thefirst reflective surface 21 a. The second intermediate image 32 in thepresent embodiment is formed so as to be separated by an air gap of37.40 mm from the second stop St2 on the enlargement side.

In this projection optical system 2 also, the first refractive opticalsystem 11 forms the first intermediate image 31 in the space Sp1 insidethe first optical system 10 and the second refractive optical system 12forms the second intermediate image 32 in the space Sp2 that is closerto the enlargement side than the first optical system 10. In theprojection optical system 2 that is non-telecentric on the incidentside, the power of the first refractive optical system 11 is designed soas to be substantially the same as or larger than the power of thesecond refractive optical system 12, and in this projection opticalsystem 2, the power of the first refractive optical system 11 is higherthan the power of the second refractive optical system 12. In theprojection optical system 2, the first intermediate image 31 and thesecond intermediate image 32 are respectively formed in regions 101 aand 101 b on opposite sides of the optical axis 101 and the light fluxthat passes the second refractive optical system 12 is concentratedabout the optical axis 101. Accordingly, the second refractive opticalsystem 12 can be formed so as to be compact.

In the projection optical system 2 also, the light ray 110 that reachesthe center of the second image plane of the screen 9 from the center ofthe first image plane formed on the DMD 5 reaches the screen 9 havingcrossed the optical axis 101 that is common to the first optical system10 and the second optical system 20 three times. The first intermediateimage 31 is formed inside the first optical system 10 and such firstintermediate image 31 is formed into the second intermediate image 32 onthe enlargement side of the first optical system 10. The second stop St2for forming the second intermediate image 32 disposes at a position thatis close to the enlargement side inside the first optical system 10.Accordingly, in the projection optical system 2 also, in the same way asthe projection optical system 1, it is possible to reduce the lensdiameter on the enlargement side of the first optical system 10 and tomake the air gap between the first optical system 10 and the firstreflective surface 21 a shorter. This means that it is possible toprovide the projection optical system 2 that is compact and can be madewide angle.

In this system 2, it is also possible to correct various aberrationssuch as curvature of field, astigmatism, and comatic aberration usingthe first refractive optical system 11 and to correct distortion such askeystone distortion using the second refractive optical system 12. Thismeans that it is possible to provide the projection optical system 2that has high performance and is capable of zooming.

In the present embodiment, the positive meniscus lens L31 that is madeof resin, is convex on the reducing side, and is disposed closest to theenlargement side of the first refractive optical system 11, that isdirectly upstream of the first intermediate image 31 has the weakestpower in the first optical system 10 and both surfaces S10 and S11 areaspherical surfaces. This means that various aberrations can befavorably adjusted by the positive meniscus lens L31 and the firstintermediate image 31 formed by enlarging the image formed by the DMD 5can be formed much more sharply.

Also, the first refractive optical system 11 tilts the firstintermediate image 31 toward the reducing side as the image heightincreases, that is as the image becomes distant from the optical axis101, so as to form the first intermediate image 31 so as to becomedistant from the positive meniscus lens L32. In addition, in the secondrefractive optical system 12, both surfaces S12 and S13 of the positivemeniscus lens L32 that is made of resin, is convex on the reducing side,and is disposed closest to the reducing side, that is, immediatelydownstream of the first intermediate image 31 have a small radius ofcurvature (that is, a large curvature). By such arrangement, keystonedistortion to the first intermediate image 31 is easy to adjust and itis possible to form the first intermediate image 31 as the secondintermediate image 32 that includes keystone distortion. In addition,since both surfaces S12 and S13 of the positive meniscus lens L32 areaspherical surfaces, it is possible to favorably correct variousaberrations aside from keystone distortion (distortion). In addition,since the positive meniscus lens L32 is convex on the DMD 5 side and theradii of curvature of both surfaces S12 and S13 are small (that is, thecurvature is large), it is possible to collect wide range of the lightemitted from the first refractive optical system 11. Accordingly, it ispossible to provide the projection optical system 2 that is bright andhas a wide angle of view.

The maximum effective diameter of the first refractive optical system 11is given by the positive meniscus lens L31 (an effective diameter of54.0 mm in the present embodiment), and the positive meniscus lens L31has the maximum effective diameter in the first optical system 10. Theeffective diameter of the negative lens L38 that is closest to theenlargement side of the second refractive optical system 12 is 26.0 mmin the present embodiment, which is smaller than the maximum effectivediameter of the first refractive optical system 11. In addition, themaximum effective diameter of the second refractive optical system 12 isgiven by the positive meniscus lens L32 (an effective diameter of 45.0mm in the present embodiment), which is smaller than the maximumeffective diameter of the first refractive optical system 11. In thisway, in the projection optical system 2 also, by having the firstrefractive optical system 11 form the image formed by the DMD 5 as thefirst intermediate image 31, it is possible to make the secondrefractive optical system 12 that is downstream of the firstintermediate image 31 smaller.

Also in this system 2, the first refractive optical system 11 forms thefirst intermediate image 31 in the first region (one region) 101 a andthe second refractive optical system 12 forms the second intermediateimage 32 in the second region (the other region) 101 b. By sucharrangement, the used region of both surfaces S12 and S13 of thepositive meniscus lens L32 is limited to the first region 101 a and theused region of the mirror 21 is limited to the second region 101 b.

Also in this system 2, the second refractive optical system 12 forms thefirst intermediate image 31 that has little distortion as the secondintermediate image 32 that is inverted in the up-down and left-rightdirections. By such arrangement, it is easy to form the secondintermediate image 32 with keystone distortion while cutting outscattered light (unneeded light). Accordingly, it is possible to providethe projection optical system 2 that is capable of projecting onto thescreen 9 enlarged and sharp images in which keystone distortion has beeneffectively cancelled out.

FIG. 7 shows lens data of the various lenses of the first optical system10 of the projection optical system 2. FIG. 8 shows various numericvalues of the projection optical system 2. In the present embodiment,since the optical distance d1 of the second intermediate image 32 on theoptical axis 101 from the second stop St2 is 53.30 mm and the opticaldistance d2 from the second stop St2 to the closest periphery (closestedge) of the second intermediate image 32 is 21.40 mm, the shownposition of the second intermediate image 32 is calculated as 37.40 mmfrom the second stop St2. Note that the distance V1 varied in zoomingindicates the air distance (gap) between the cover glass CG and thefirst lens group G1, the distance V2 indicates the air gap between thefirst lens group G1 and the second lens group G2, and the distance V3indicates the air gap between the second lens group G2 and the thirdlens group G3.

FIG. 9 shows the light flux in the peripheries of the first intermediateimage 31 and the second intermediate image 32 of the projection opticalsystem 2 by way of spot diagrams. It can be understood that the secondintermediate image 32 includes keystone distortion and that keystonedistortion is corrected in the first intermediate image 31.

For the projection optical system 2 of this embodiment, Conditions (1)to (5) described above are as indicated below.EXP/dw=0.28  Condition (1)dn/dw=0.25  Condition (2)MD/ID=4.3  Condition (3)LLD/ID=1.0  Condition (4)STD2/MD=16.6  Condition (5)Accordingly, the projection optical system 2 of this embodiment alsosatisfies Conditions (1) to (5).

By arranging the thirteen lenses L11 to L13, L21 to L22, and L31 to L38and the single mirror 21 as above, the projection optical system 2according to the second embodiment is a high performance,non-telecentric projection optical system that is comparatively brightwith an F number of 2.62, is capable of zooming, is wide angle with amaximum angle of view (full angle) of 75.34 degrees and a focal lengthof 3.63 at the wide-angle end, and is capable of projecting sharpimages.

Third Embodiment

FIG. 10 shows a projection optical system 3 according to a thirdembodiment. The projection optical system 3 is telecentric on theincident side and is capable of zooming. The projection optical system 3includes, in order from the DMD 5 side (reducing side), the firstoptical system 10 that includes a plurality of lenses and the secondoptical system 20 that includes the first reflective surface 21 a thatreflects light emitted from the first optical system 10 to project thelight onto the screen 9 on the enlargement side. The first opticalsystem 10 includes sixteen lenses L11, L21 to L26, L31 to L32, and L41to L47 and the second optical system 20 includes the mirror (curvedmirror) 21 that has an aspherical first reflective surface 21 a.

The first optical system 10 carries out zooming and includes, in orderfrom the DMD 5 side, the first lens group G1 that has positiverefractive power, the second lens group (former group) G2 that haspositive refractive power, the third lens group (middle group) G3 thathas positive refractive power and a fourth lens group (latter group) G4that has positive refractive power. The first optical system 10 of thisembodiment also forms the first intermediate image 31 that has beenformed inside the first optical system 10 into the second intermediateimage 32 closer to the enlargement side than the first optical system10, and includes the first refractive optical system 11 with negativepower that forms the image formed by the DMD 5 as the first intermediateimage 31 and the second refractive optical system 12 with positive powerthat forms the first intermediate image 31 into the second intermediateimage 32. The first intermediate image 31 is formed inside the fourthlens group G4 and the first lens group G1, the second lens group G2, thethird lens group G3, and the first (reducing side) lens L41 of thefourth lens group G4 consist of the first refractive optical system 11and the other lenses of the fourth lens group G4 consist of the secondrefractive optical system 12.

The first lens group G1 closest to the DMD 5 (i.e., closest to thereducing side) has, as a whole, positive refractive power. The firstlens group G1 consists of the positive meniscus lens L11 that is convexon the enlargement side. On the DMD 5 side of the first lens group G1, asingle cover glass CG made of glass and a single TIR prism Pr that isalso made of glass are disposed in that order from the DMD 5 side, andthe projected light from the DMD 5 enters the projection optical system3 in a telecentric or near-telecentric state.

The second lens group (former group) G2 has, as a whole, positiverefractive power. The second lens group G2 consists of a biconvexpositive lens L21, a first cemented lens (balsam lens, doublet) LB1where two lenses are stuck together, a second cemented lens LB2 wheretwo lenses are stuck together, and a positive meniscus lens L26 that isconvex on the enlargement side disposed in that order from the DMD 5side. The first cemented lens LB1 is constructed of a biconvex positivelens L22 and a biconcave negative lens L23 that are disposed in thatorder from the DMD 5 side. The second cemented lens LB2 is constructedof a biconcave negative lens L24 and a biconvex positive lens L25 thatare disposed in that order from the DMD 5 side. The surface on theenlargement side of the positive lens L25 is an aspherical surface. Thefirst aperture stop St1 is disposed on the enlargement side of thepositive lens L25, that is, in the space between the positive lens L25and the positive meniscus lens L26.

The third lens group (middle group) G3 has as a whole positiverefractive power. The third lens group G3 consists of a biconcavenegative lens L31 and a biconvex positive lens L32 disposed in thatorder from the DMD 5 side.

The fourth lens group (latter group) G4 that is closest to theenlargement side has as a whole positive refractive power. The fourthlens group G4 consists of a positive meniscus lens L41 that is biconvexand made of resin, a positive meniscus lens L42 that is convex on theDMD 5 side and made of resin, a biconvex positive lens L43, a thirdcemented lens LB3 where two lenses are stuck together, and a fourthcemented lens LB4 where two lenses are stuck together disposed in thatorder from the DMD 5 side. The third cemented lens LB3 is constructed ofa biconvex positive lens L44 and a biconcave negative lens L45 that aredisposed in that order from the DMD 5 side. The fourth cemented lens LB4is constructed of a biconcave negative lens L46 and a biconvex positivelens L47 that are disposed in that order from the DMD 5 side. Bothsurfaces of the positive meniscus lens L41, that is, the surface S17 onthe reducing side and the surface S18 on the enlargement side, areaspherical surfaces. In addition, both surfaces S19 and S20 of thepositive meniscus lens L42 are aspherical surfaces. The second aperturestop St2 is disposed on the enlargement side of the fourth lens groupG4.

In this projection optical system 3, when zooming from the wide angleend to the telephoto end, the first lens group G1 does not move, thesecond lens group (former group) G2 moves from the reducing side to theenlargement side, the third lens group G3 moves from the enlargementside to the reducing side so as to compensate for the movement of thesecond lens group G2 so that the light flux incident on the fourth lensgroup G4 that is the relay lens satisfies certain conditions. The fourthlens group (latter group) G4 does not move. The projection opticalsystem 3 is a floating focus or inner focus optical system that adjuststhe focus point inside the fourth lens group G4 that does not moveduring zooming. Focusing in the present embodiment is carried out bymoving at least one lens included in the fourth lens group G4.

The first refractive optical system 11 configured by the first to thirdlens groups G1 to G3 and the lens L41, is a lens system that as a wholehas negative refractive power and includes a positive meniscus lens L11,a positive lens L21, a first cemented lens LB1, a second cemented lensLB2, a first stop SU, a positive meniscus lens L26, a negative lens L31,a positive lens L32, and a positive meniscus lens L41 disposed in thatorder from the DMD 5 side. The first intermediate image 31 is formedcloser to the enlargement side than the first refractive optical system11, that is, in a space Sp1 between the first refractive optical system11 and the second refractive optical system 12. The first intermediateimage 31 in the present embodiment is formed so as to be separated by adistance of 1.00 mm from the positive meniscus lens L41 on theenlargement side of the positive meniscus lens L41.

The second refractive optical system 12 configured by the remaininglenses L42 to L47 of the fourth lens group G4, is a lens system that asa whole has positive refractive power, and includes a positive meniscuslens L42, a positive lens L43, the third cemented lens LB3, the fourthcemented lens LB4, and the second stop St2 disposed in that order fromthe DMD 5 side. The second intermediate image 32 is formed closer to theenlargement side than the second refractive optical system 12, that is,in the space Sp2 between the second refractive optical system 12 and thefirst reflective surface 21 a. The second intermediate image 32according to the present embodiment is formed so as to be separated fromthe second aperture stop St2 by a distance of 33.80 mm on theenlargement side of the second aperture stop St2.

In the projection optical system 3 also, the first refractive opticalsystem 11 forms the first intermediate image 31 in the space Sp1 insidethe first optical system 10 and the second refractive optical system 12forms the second intermediate image 32 in the space Sp2 that is closerto the enlargement side than the first optical system 10. In theprojection optical system 3 that is telecentric on the incident side,the power of the first refractive optical system 11 is weaker than thepower of the second refractive optical system 12, the first intermediateimage 31 and the second intermediate image 32 are shifted toward theenlargement side compared to the projection optical system 2 that isnon-telecentric, and the first intermediate image 31 and the secondintermediate image 32 are respectively formed in the regions 101 a and101 b on opposite sides of the optical axis 101. Accordingly, the lightflux that passes the second refractive optical system 12 is furtherconcentrated around the optical axis 101 and it is possible to make thesecond refractive optical system 12 much more compact.

In the projection optical system 3 also, the light rays 110 that reachthe center of the second image plane of the screen 9 from the center ofthe first image plane formed on the DMD 5 reach the screen 9 havingcrossed the optical axis 101 that is common to the first optical system10 and the second optical system 20 three times. This means that even ifthe enlargement side of the first optical system 10, that is, the secondrefractive optical system 12, is constructed by lenses that haverotational symmetry about the optical axis 101, it is possible tosuppress interference with the projected light reflected by the firstreflective surface 21 a that has rotational symmetry around the opticalaxis 101 and it is possible to emit light rays from the first reflectivesurface 21 a closer to the optical axis 101. That is, it is possible toeffectively use the region close to the optical axis 101 of the firstreflective surface 21 a that has rotational symmetry, and possible toenlarge and project images with a large angle of view onto the screen 9with a small elevation angle with respect to the optical axis 101.

In addition, since the first intermediate image 31 is formed inside thefirst optical system 10 and the first intermediate image 31 is formed asthe second intermediate image 32 on the enlargement side of the firstoptical system 10, the second stop St2 for forming the secondintermediate image 32 can be disposed closer to the enlargement sidethan the first optical system 10. Accordingly, in the projection opticalsystem 3 also, in the same way as the projection optical system 1, it ispossible to make the lens diameter on the enlargement side of the firstoptical system 10 smaller and to make the air gap between the firstoptical system 10 and the first reflective surface 21 a smaller. Thismeans that it is possible to provide the projection optical system 3that is compact and can project wider angle.

Also in this projection optical system 3, the first refractive opticalsystem 11 corrects various aberrations such as curvature of field,astigmatism, and comatic aberration and the second refractive opticalsystem 12 corrects distortion such as keystone distortion, the system 3has high performance, and is capable of zooming. In the first opticalsystem 10, both surfaces S17 and S18 of the positive lens L41 that isbiconvex, is made of resin, and is closest to the enlargement side ofthe first refractive optical system 11 are aspherical surfaces, and itis possible to sharply form the first intermediate image 31 produced byenlarging the image formed by the DMD 5. Both surfaces S19 and S20 ofthe positive meniscus lens L42 that is convex on the reducing side andis disposed closest to the reducing side of the second refractiveoptical system 12 have small radii of curvature (that is, a largecurvature), and the surface S20 on the mirror 21 side has the nextsmallest radius of curvature after the surface S19 (that is, a largecurvature). This means that this optical system makes it easy to formthe first intermediate image 31 that is formed so as to be slightlyinclined toward the reducing side into the second intermediate image 32that has keystone distortion. Accordingly, it is possible to project animage which is sharp and in which keystone distortion has been correctedonto the screen 9.

The effective diameter of the positive lens L41 of the first refractiveoptical system 11 is 49.0 mm and is the lens that gives the maximumeffective diameter of the first refractive optical system 11 and thefirst optical system 10. The effective diameter of the positive lens L47that is closest to the enlargement side of the second refractive opticalsystem 12 is 11.0 mm in the present embodiment and is smaller than themaximum effective diameter of the first refractive optical system 11. Inaddition, the effective diameter of the positive meniscus lens L42 thatgives the maximum effective diameter of the second refractive opticalsystem 12 is 32.0 mm and the maximum effective diameter of the secondrefractive optical system 12 is smaller than the maximum effectivediameter of the first refractive optical system 11.

Since the second refractive optical system 12 of this system also formsthe first intermediate image 31 that has little distortion as the secondintermediate image 32 that is inverted in the up-down and left-rightdirections, it is possible to provide the projection optical system 3that facilitates formation of the second intermediate image 32 that haskeystone distortion while cutting out scattered light (unneeded light)and is also capable of projecting, onto the screen 9, a sharp andenlarged image in which keystone distortion is effectively cancelledout.

FIG. 11 shows lens data of the various lenses of the first opticalsystem 10 of the projection optical system 3. FIG. 12 shows variousnumeric values of the projection optical system 3. In the presentembodiment, since the optical distance d1 of the second intermediateimage 32 on the optical axis 101 from the second stop St2 is 59.00 mmand the optical distance d2 of the closest periphery (closest edge) ofthe second intermediate image 32 from the second stop St2 is 8.50 mm,the shown position of the second intermediate image 32 is calculated asa position 33.80 mm from the second stop St2. Note that the distancesvaried during zooming V1-V3 indicate the air gaps between the first lensgroup G1 and the second lens group G2, the second lens group G2 and thethird lens group G3, and the third lens group G3 and the fourth lensgroup G4, respectively. FIG. 13 shows the light flux on planes in theperipheries of the first intermediate image 31 and the secondintermediate image 32 of the projection optical system 3.

For the projection optical system 3, Conditions (1) to (5) describedabove are as shown below.EXP/dw=0.34  Condition (1)dn/dw=0.25  Condition (2)MD/ID=3.8  Condition (3)LLD/ID=0.4  Condition (4)STD2/MD=9.2  Condition (5)Accordingly, the projection optical system 3 according to the presentembodiment satisfies Conditions (1) to (5).

By being configured of the sixteen lenses L11, L21 to L26, L31 to L32,and L41 to L47 and the single mirror 21, the projection optical system 3according to the third embodiment is a high performance, telecentricprojection optical system that is comparatively bright with an F numberof 2.43, is capable of zooming, is wide angle with a maximum angle ofview (full angle) of 75.32 degrees and a focal distance of 3.46 at thewide-angle end, and is capable of projecting sharp images.

Note that the present invention is not limited to such embodiments andis defined by the scope of the claims. The optical systems describedabove are examples and the surfaces of the lenses and/or the mirrorsurface (reflective surface) included in the projection optical systemmay be spherical surfaces or aspherical surfaces with rotationalsymmetry or may be asymmetrical surfaces, for example, free-formsurfaces. At least one of the lenses included in the first opticalsystem and/or the reflective surface included in the second opticalsystem may be off-center from the optical axis. In this case, theoptical axes of the respective optical systems include the optical axesof the main optical elements. Also, the optical axis of the firstoptical system and the optical axis of the second optical system may bethe same or may be off-center (shifted). The light valve 5 may be athree panel-type light modifier 5 that splits a white light source intothree colors using a dichroic filter (mirror) or the like, and the lightmodifier 5 may be an LCD (liquid crystal panel), a self-luminous organicEL, or the like. The first optical system 10 and the second opticalsystem 20 may further include a prism or mirror that bends the opticalpath. For example, it is possible to dispose one or a plurality ofmirrors or prisms on the reducing side or the enlargement side of thefirst reflective surface 21 a. It is also possible to further include arefractive optical system on the enlargement side of the second opticalsystem 20.

The invention claimed is:
 1. A projection optical system that projectsfrom a first image plane on a reducing side to a second image plane onan enlargement side, comprising: a first optical system including afirst refractive optical system and a second refractive optical system,the first refractive optical system including a plurality of lenses andforms light incident from the reducing side into a first intermediateimage on the enlargement side, the second refractive optical systemincluding a plurality of lenses and forms the first intermediate imageon the reducing side into a second intermediate image on the enlargementside; a first reflective optical system that includes a first reflectivesurface with positive refractive power that is positioned closer to theenlargement side than the second intermediate image; a first stopdisposed between the first image plane and the first intermediate image;and a second stop disposed between the first intermediate image and thesecond intermediate image, and wherein an optical distance EXP betweenan exit pupil and the first reflective surface when the first stop is astop of the first optical system and an optical distance dw between thefirst image plane and the first reflective surface satisfy the followingcondition0.1<EXP/dw<0.6.
 2. The projection optical system according to claim 1,wherein the first refractive optical system forms an image formed on thefirst image plane into the first intermediate image, and a light raythat reaches a center of the second image plane from a center of thefirst image plane reaches the second image plane having crossed any ofan optical axis of the first refractive optical system, an optical axisof the second refractive optical system, and an optical axis of thefirst reflective optical system three times.
 3. The projection opticalsystem according to claim 1, wherein the first image plane and the firstreflective surface are disposed in a same side with respect to a firstplane that includes an optical axis.
 4. The projection optical systemaccording to claim 1, wherein an effective diameter of a lens that isclosest to the enlargement side of the second refractive optical systemis smaller than a maximum effective diameter of the first refractiveoptical system.
 5. The projection optical system according to claim 1,wherein a maximum effective diameter of the second refractive opticalsystem is smaller than a maximum effective diameter of the firstrefractive optical system.
 6. The projection optical system according toclaim 1, wherein the first refractive optical system is an equalmagnification or a magnifying optical system.
 7. The projection opticalsystem according to claim 1, wherein the second stop is an eccentricstop.
 8. The projection optical system according to claim 1, wherein thefirst intermediate image and the second intermediate image are formed onopposite sides of the optical axis.
 9. The projection optical systemaccording to claim 1, wherein the first optical system includes a zoomoptical system.
 10. The projection optical system according to claim 9,wherein the first optical system includes, in order from the reducingside, a former group with positive refractive power, a middle group withpositive refractive power, and a latter group with positive refractivepower, and when zooming from a wide angle end to a telephoto end, theformer group moves from the reducing side to the enlargement side, themiddle group moves so as to compensate for movement of the former group,and the latter group is fixed, and the first intermediate image isformed inside the latter group.
 11. A projector comprising: theprojection optical system according to claim 1; and a light modulatorthat forms an image on the first image plane.
 12. The projectoraccording to claim 11, further comprising an illuminating optical systemthat illuminates the first image plane and is disposed in a same side asthe first reflective surface with respect to a first plane that includesthe optical axis.
 13. A projection optical system that projects from afirst image plane on a reducing side to a second image plane on anenlargement side, comprising: a first refractive optical system thatincludes a plurality of lenses and forms light incident from thereducing side into a first intermediate image on the enlargement side; asecond refractive optical system that includes a plurality of lenses andforms the first intermediate image on the reducing side into a secondintermediate image on the enlargement side; and a first reflectiveoptical system that includes a first reflective surface with positiverefractive power that is positioned closer to the enlargement side thanthe second intermediate image; wherein an optical distance do betweenthe lens closest to the enlargement side of the second refractiveoptical system and the first reflective surface and an optical distancedw between the first image plane and the first reflective surfacesatisfy the following condition0.1<dn/dw<0.3.
 14. The projection optical system according to claim 13,wherein the first refractive optical system forms an image formed on thefirst image plane into the first intermediate image, and a light raythat reaches a center of the second image plane from a center of thefirst image plane reaches the second image plane having crossed any ofan optical axis of the first refractive optical system, an optical axisof the second refractive optical system, and an optical axis of thefirst reflective optical system three times.
 15. The projection opticalsystem according to claim 13, wherein the first image plane and thefirst reflective surface are disposed in a same side with respect to afirst plane that includes an optical axis.
 16. The projection opticalsystem according to claim 13, wherein an effective diameter of a lensthat is closest to the enlargement side of the second refractive opticalsystem is smaller than a maximum effective diameter of the firstrefractive optical system.
 17. The projection optical system accordingto claim 13, wherein the first intermediate image and the secondintermediate image are formed on opposite sides of an optical axis. 18.The projection optical system according to claim 13, further comprisinga first optical system including the first refractive optical system andthe second refractive optical system, and wherein the first opticalsystem includes a zoom optical system.
 19. A projector comprising: theprojection optical system according to claim 13; and a light modulatorthat forms an image on the first image plane.
 20. The projectoraccording to claim 19, further comprising an illuminating optical systemthat illuminates the first image plane and is disposed in a same side asthe first reflective surface with respect to a first plane that includesthe optical axis.